The present invention relates to a torque sensor in; for instance, an electric power steering system, used for detecting steering torque applied to a steering member for steering purpose.
In an electric power steering system that drives a steering assist motor in accordance with turning operation of a steering member, such as a steering wheel, and that transmits rotary force of the motor to a steering mechanism, to thus assist steering operation, there is a necessity for detecting steering torque applied to the steering member for the purpose of being used for controlling driving of the steering assist motor. For the detection, a torque sensor that is disposed at any position on a steering shaft and that establishes mutual communication between the steering member and a steering mechanism has been used.
The torque sensor is configured that the steering shaft, which is to serve as an object of detection, is divided into first and second shafts coaxially linked by a torsion bar which serves as a torsion spring and which has a small diameter; such that relative angular displacement arises between the first and second shafts in conjunction with a twist of the torsion bar when steering torque is applied to the steering shaft by rotating operation of the steering member; and such that the steering torque is detected by taking the relative angular displacement as a medium.
The relative angular displacement between the first and second shafts has been detected by various related devices. JP-A-2003-149062 discloses a torque sensor that utilizes changes in a magnetic circuit provided existing between a cylindrical magnet and a pair of magnetic yokes. The cylindrical magnet rotates integrally with a first shaft. The magnetic yokes rotate integrally with a second shaft.
The magnetic yokes that rotate integrally with the second shaft correspond to rings made of a soft magnetic substance. The rings have a plurality of pole claws which extend in an axial direction toward one side of an annular yoke main body. The pole claws of each ring are spaced at an equal distance along a circumferential direction. The respective pole claws are alternately positioned in the circumferential direction, such that the respective rings are arranged in an axially-longitudinal direction and fixed to the second shaft. Further, the cylindrical magnet that rotates integrally with the first shaft is a multipole magnet having pairs of magnetic poles which are equal in number to the pole claws of the magnetic yokes and which are arranged side by side along the circumferential direction. The cylindrical magnet is fixed to the first shaft while phase adjustment is achieved along the circumferential direction in such a way that the pole claws of the magnetic yokes conform with borders among the north and south poles in a neutral state where relative angular displacement does not arise in the first and second shafts.
Magnetism collection rings made of a soft magnetic substance are disposed outside the two magnetic yokes so as to be in close proximity and opposite respective yoke main bodies. These magnetism collection rings have magnetism collection sections that are arranged in a line and that oppose each other with a predetermined air gap therebetween. Magnetic sensors using magnetic sensing elements, such as Hall elements, are arranged in the respective air gaps among the magnetism collection sections.
By the foregoing configuration, when relative angular displacement arises between the first and second shafts, phase differences of opposite directions develop between the pole claws of the two magnetic yokes and the magnetic poles of the cylindrical magnet. By changes in magnetic flux in the respective different magnetic yokes responsive to the phase difference, the magnetic fluxes leaking to the air gaps among the magnetism collection sections of the respective magnetism collection rings increase or decrease. The relative angular displacement developing between the first and second shafts can be detected by extracting the changes in the output from the magnetic sensor conforming to such an increase or decrease, and torque (steering torque) applied to the first and second shafts can be determined.
When the torque sensor configured as mentioned above is applied to an electric power steering system, countermeasures against a failure are indispensable for eliminating the potential of steering assistance becoming unstable as a result of erroneous detection of the steering torque.
In the torque sensor described in the prior art, two magnetic sensors are arranged along the circumference of the magnetism collection ring, and determination of a failure in each of the magnetic sensors is consecutively performed by comparing outputs from the sensors. Even when one of the magnetic sensors is determined to be failed, it is still possible to detect torque by an output from the other magnetic sensor, thereby enabling continuation of a steering assist. There has also been proposed a torque sensor having three magnetic sensors, wherein outputs from the sensors are compared with each other, to thus facilitate location of a failed magnetic sensor based on majority rule.
However, determination of a failure, such as that mentioned above, is performed by taking a failure or anomaly in each of the magnetic sensors as an object of detection. In contrast, a failed state where a normal torque detection value is not acquired is also induced by a shortcircuit in signal lines of the respective magnetic sensors. In this case, it is difficult to make a determination by comparing outputs to each other. When the torque sensor is used as a device for detecting steering torque in an electric power steering system, an erroneous steering assist will be performed on the basis of a torque detection value acquired with the signal lines being short-circuited, which raises a problem of the driver feeling a sense of discomfort.